Method of making screw-cutting dies and the like



Feb. 6, 1923.

P. W. MARTIN METHOD OF MAKING SCREW CUTTING DIES AND THE LIKE Filed Oct. 21, 1920 2 sheets-sheet l w ATTORNEY Feb. 6, 1923. 11,444,424

' P. W. MARTIN METHOD OF MAKING SCREW CUTTING DIES AND THE LIKE Filed Oct. 21, 1920 2 sheets-sheet 2 DIA NaoF m g gI'ALDOF TgggL P ggla E E OF PIPE THREAD INCHES LAD SNEWEWN fl i m (M RN V .840 1+ 6 3%, P L050 4.20 [6 L315 u 5.26 :6 L660 m? 6.64-- is Z-ZrP L990 7.60 16 2.375 I6 5: 4P 2.875 I 1e 4 P INVENTOR BY 1 ATTORNEY PERCY W. M'AR'IIN, 01E BAY CITY, MICHIGAN, ASSIGNOR TO SMALLEY GENERAL 00., 0F BAY CITY, MICHIGAN, A CQRPORATIOH Q1 MICHIGAN.

METHOD OF MTAKING: SCREW-CUTTING DIES AND THE LIKE.

Application filed October 21, 1920.

To all whom it may concern:

Be it known that l, Pnnor W. lilsnrin, a citizen of the United States, residing at Bay City, in the county of Bay and State of Michigan, have invented certain new and useful Improvements in Methods of Making Screw-Cutting Dies and the like; and I do hereby declare the following to be a full, clear, and exact description of the inven tion, such as will enable others skilled in the art to which it appertains to make and use the same.

This invention relates to a method for forming screw-cutting dies or chasers and pertains more particularly to a method whereby I am enabled to form at one operation a plurality of sets of screw-cutting dies, each die of any one of said sets of dies being interchangeable with the corresponding die of any other set.

With the foregoing and certain other ob jects in view which will appear later in the specification, my invention consists in the method herein described and claimed and the equivalents thereof.

Heretofore chasers for dies of two and one-half inch diameter and larger have been formed by milling. Smaller diameters have not usually been milled because the milling hob had to be so small that it did not possess the proper strength and rigidity.

If the chasers intended for a die of small diameter were attempted to be milled by arranging them at the quarter points of a larger diameter circle and then milling with a hob of the correct pitch, sets of dies of the correct pitch would be obtained, but the helical angle would be incorrect.

Therefore, to produce a correct set of dies for small diameter work by milling the dies while arranged on a circle of larger diameter, it is necessary to employ the step in my method which consists in imparting tothe hob a sufficiently increased movement of lead to produce the correct helical angle on the dies, as will be later described.

My invention applies the method of milling to the forming of screw-cutting dies adapted to small diameter work and it also.

enables several such sets of dies to be threaded at one operation, by a milling cutter which is of large diameter, even larger than the diameter of the work.

Serial No. 418,371.

The fundamental requirement for an accurate set oi screw-cutting dies is that the die threads shall. have the proper pitch, the proper lead and the proper helical angle, and that the cutting die shall have the proper clearance or back-oil. My improved method produces dies possessing all of these qualities.

In carrying out my method, I employ the usual face-plate and lead-screw mechanism of the ordinary milling machine or screwcutting lathe, or an equivalent machine, as the case may be.

I take several sets of die blanks and arrange them on a faceplate in circular arrangement, the diameter of the circle of out being several times larger than the diameter or" the work. i

I cut the die teeth by means of the usual multiple thread milling hob or cutter.

In the drawings, Fig. 1 is a diagrammatic front view of the face-plate of a die-threading machine, showing a plurality 01"- sets of die blanl rs in place and also showing the cutting hob for forming the die teeth.

F 1g. 2 is a pan sectional View, taken on the line 2-2 of Fig. 1.

Fig. 3 is a tabulation showing the dimen sions of iron pipes of various sizes and the required diameter of cutting circle and the required lead.

Fig. 4- is a diagrammatic View, not to scale, showing the means of determining the amount of lead movement required to be given to the hob in order that each resulting set 01" dies may when assembled produce on small diameter work a thread having the required standard pitch.

Fig. 5 is a diagrammatic view showing the work being threaded by a die, formed by my improved method, and showing the relist or backing-oil clearance imparted to the finished die. I

is is'clearly Shown in the drawings, my method may be carried out by employing the usual face-plate 1 and hie cutting may be done by the usual multiple toothed hob 2, the faceplate and hob driven in the usual manner. The hob is adapted to be given an axial movement of lead, by means of lead screw or other device well-known in the art and consequently not shown in the drawing I form the cutting teeth on the dies 3 by the following method.

If the size of pipe to be threaded, as indicated diagrammatically at W, Fig. 5, is for example, one-half inch, and the pitch of the finished standard pipe thread 5 is, say, fourteen threads to the inch, the outside diameter of the pipe is .840 inch.

Assuming now that I desire to make four sets of these small diameter dies at one op eration of the milling machine, I divide the circle of the face-plate-see Fig. 1into four quarters and arrange the four die blanks of a set, equally spaced, one set in each quarter, as shown at R, S, T, U. The diameter of this cutting circle must bear a predetermined relation to the diameter of the work which in the assumed case is one-half inch. 1, therefore, find the proper diameter of the milling circle by multiplying the diameter of the work, in the case illustrated .840 inch, by the number of sets of dies arranged on the circle, viz four. Hence .840 inch times 4- equals inch, which is the proper diameter of the milling circle.

The pitch of a standard one-half inch pipe thread is fourteen threads to the inch. The pitch of the teeth of the milling cutter is made fourteen to the inch, but the lead or axial movement imparted to the milling hob during one revolution of the face-plate.

must be four times one-fourteenth or twosevenths inch, corresponding to three and 0ne-half pitch, as shown by the table, Fig. 3, and by the diagram, Fig. l.

That part of the base of the triangle marked rev. equals the circumference of the work and the vertical line P equals the pitch of the thread. The diagonal line represents the developed thread and the angle between this line and the base line is the helical angle.

By extending the base line to make its length equal four times the circumference of the work or 1 rev., and extending the diagonal line and projecting a vertical line from the end of the base line until it intersects the diagonal, the length 4P of the vertical line will equal four times the pitch P.

Therefore, to mill four sets of small diameter dies R, S, T, U on a large diametercircle it is necessary to impart to the hob or milling cutter a lead equal to four times the pitch. The actual lead movement of the hob in cutting each set of dies, as in Fig. 1, will then beexactly equal to the pitch.

All four sets of cutting dies are threaded during one revolution of the face-plate. The helical angle of the threads so formed on each set of dies R, S, T, U is exactly the same as if the four dies of each set had been arranged in a small circle equal in diameter to the diameter of the work, and then tapped by a standard fourteen pitch tap. Thus, by arranging a plurality of sets Referring to Fig. 5,

depth of the thread in the work and g is the radius of the milling circle. Therefore, the distance C represents the clearance at the top of the thread, that is, the amount the die 3 is relieved or backed off when the die is used to cut a pipe of diameter 7.

Although a plurality of sets, as R, S, T, U may be made at one operation by the here'- in described method, it is also frequently desirable that on the next operation of the machine the resulting sets may be exact duplicates of the first sets. To accomplish this result and to produce an indefinite number of interchangeable sets it is only necessary to start the cutting hob in each case on the corresponding cutter, as for example, the first cutter of set R, as shown in Fig. 1, and to insure that at each operation the position of the hob longitudinally with respect to the said die is the same. This can easily be accomplished by inserting a shim between the end of the hob and the face-plate, as at 6 in Fig. 2, so that at the commencement of each successive operation the longitudinal position of the hob will be identical with re spect to the position of the first die of the set.

While I have shown and described a certain type of face-plate and hob for forming screw-cutting dies, it will be understood that any other type of machine might be employed for this purpose. My improved method is not dependent upon the employment of any certain type of machine.

For purpose of illustration and explanation I have described a face plate spaced to accommodate four sets of dies. It will be understood, however, that one of these four sets, or more than one, might be omitted from its assigned space without interfering with the correct cutting of the remaining sets and without departing'from my invention as set forth in the claims.

It will also be noticed that I have-described one set of dies, as R, S, T, U, each set consisting of four cutters, but it will be observed that if desired the first two cutters of, say, set S, could be mounted in the space allotted to that set and the last two cutters of set S could be mounted in the third and fourth posit-ions of set It and the fourcutters thus formed would comprise a complete set.

Similarly any other suitable or desirable arrangement or division of the sets of cutters may be made Without departing from my invention as set forth in the claims.

Having thus described my invention, What I claim and desire to secure by Letters Patent is:

1. The herein described method of forming at one operation a plurality of sets of screw-cutting dies adapted to cut Wort of small diameter, said method comprising arranging a plurality of sets of die blanks on a milling circle as many times the diameter of the Work upon Which the dies are to be used as there are sets of dies, forming teeth on said blanks by milling, and simultaneously imparting to the milling cutter and Work a relative longitudinal movement such that the milling cutter during one revolution of the Work mills a thread having a lead as many times the pitch of a set of dies as there are sets of dies.

2. The herein described method of forming at one operation a plurality of sets of screw cutting dies comprising arranging a plurality of sets of die blanks in circular form on a milling circle as many times the diameter of the Work upon Which the dies are to be used as there are sets of dies, forming teeth on said blanks by milling and simultaneously imparting to the'milling cutter a movement of lead as many times the pitch of a set of dies as there are sets of, dies, whereby to produce in the Work a thread having the required helix angle.

8. The herein described method of torming at one operation a plurality of sets of screw cutting dies adapted to out work of small diameter, said method comprising arranging spaces for a plurality of sets of die blanks on a milling circle as many times the diameter of Work upon which the dies are to be used as there are spaces for sets of die blanks, placing die blanks in one or more of said spaces, forming teeth on said blanks by milling, and simultaneously imparting to the milling cutter and work a relative longitudinal movement such that the milling cutter during one revolution of the Work mills a thread having a lead as many times the pitch of a set of dies as there are spaces for sets of dies.

In testimony whereof, I afiix my signature.

PERCY W. MARTIN. 

